1. Field of the Invention
The present invention relates to a printed-circuit board having projection electrodes which is mounted with semiconductor devices or other electronic components and a method for producing such a printed-circuit board.
2. Prior Art
Printed-circuit boards have packaging electrodes for electrically connecting one or more semiconductor devices and other electronic components mounted thereon. These printed-circuit boards are multilayered and formed with electric conductors, particularly, inter-connectors which are connected to the surface electrodes and interposed between the inner insulating layers of the multilayer structure, allowing highly densified electronic components to be mounted.
The miniaturization and high-speed performance for packaging the semiconductor devices for recent electronic devices have led to a significant tendency, to use a package free from leads, which is formed with grid array connecting electrodes on the backface of the package, as called ball grid array (BGA). Because such a package, which differs from a quadflat package (QFP), has no connecting leads extending externally, miniaturized packaging and high-speed performance can be attained.
For instance, FIG. 10 shows a land grid array package 101 in which a semiconductor chip 9 is mounted on a carrier substrate 103 (see J. Nikkei Electronics, Aug. 2 (1993) pages 104-118).
For packaging the above BGA, balls 111 composed of a solder are attached on the package 103 in an array of each terminal electrode 102 of the package which is mounted on a printed circuit board (not shown) having electrodes with the same array as the terminal electrodes through the solder balls 111. In this package, the solder balls 111 are made larger in diameter whereby the package 103 mounted on the printed-circuit board is increased in height. This relieves a shearing stress produced by mismatching or displacement of the package 101 relative to the printed-circuit board caused by thermal expansion due to changes in atmospheric temperature.
The projection electrodes including bumps such as the above solder balls are formed on a printed circuit board, not to the package of a semiconductor device or electronic components. It is advantageous in view of mass production and cost that the projection electrodes are attached to the side of the printed-circuit board.
There is known ceramic substrates having a structure in which the above package electrode projects from the surface of the substrate. Such a structure is disclosed in Japanese Patent Publication JP-A 8-88470. In the substrate, two or more green sheets of insulating ceramics material are laminated with conductive patterned foils between the sheets and thorough-holes opened on the surface of the laminated body. Then, a film, which is easily burned out at a low temperature, is covered on the multi-layer laminated body. The film is previously formed with through holes corresponding to the terminal electrodes formed on the bottom of an integrated circuit, for example, and filed with a conducting connecting material into the holes and laminated by pressing. Thereafter the substrate is then fired to burn out the film, thereby forming projection electrodes by sintering the filled material in the holes.
Japanese Patent Publication JP-A 7-202422 discloses a method in which for forming projection electrodes a ceramic film having a high softening point with a plurality of through-holes is formed on at least one surface of a laminated ceramic green sheets. The through-holes are filled with a conducting paste. As the laminated body is fired to be sintered without sintering the film which is delaminated off to leave projection electrodes on the substrate. For the film for forming the projection electrode, a material such as aluminum powder which can never be sintered at the firing temperature of the ceramic green sheet is utilized, thereby facilitating the peeling after sintering. The projection electrodes composed of the metal differing from the conductive material for the via hole, and, for example, silver is preferably utilized.
As the ceramic substrate having such projection electrodes is used for mounting the electronic components, a bonding defect and cracks in the rigid projection electrode are caused by their deformation stresses due to mismatching of these numerous electrodes of the electronic components with the same number of projection electrodes bonded onto the above substrate.
As for the above-mentioned synthetic resin substrate, FIG. 12 shows a conventional example of the projection electrodes formed on a printed-circuit board. First, a printed-circuit board is formed using a glass fiber reinforced epoxy resin substrate 3, interlayer via hole conductors 2 by metallizing the through-holes, and patterned electrodes 3 of a copper foil. Thereafter, connecting projection electrodes 121, which are tens of micrometers in height and is composed of a metal such as Au, Ag, Cu, or a solder alloy, are formed on electrode portions of the substrate by means of a plating method or screen printing method using a mask.
The printed-circuit board with the projection electrode is mounted with semiconductor devices or other electronic components to constitute a multichip module (MCM) or utilized as a motherboard for mounting bare chips.
Though not for packaging projection electrodes, a technology is disclosed in Japanese Patent Publication JP A 7-147464 in which a multilayer resin substrate is formed by lamination using a plurality of resin prepregs each of which conducting pasts are projected on the surface. Particularly, in this technology, parting films is applied to both faces of each of the resin prepreg, having the through holes which are filled with a conductive paste, and peeled off as the past is uncured to form the prepreg with the filled paste projecting from the both faces thereof. A prescribed number of the prepregs and the patterned copper foils are alternatively multiplied into a lamination, and then both lamination faces are pressed under heat to cure the prepregs, thereby securing the connection between conductors in each layer in the substrate.
However, in above-mentioned land grid array-type printed-circuit board made of a synthetic resin, the packaging projection electrodes have been formed by a screen printing or metal plating method on the corresponding electrodes formed from patterned metal foils after the printed board is cured. In this case, the adhesive strength between the projection electrode and the electrode on the printed-circuit board is low because of different materials and different formation processes. Also, in such a printed-circuit board formed with via holes, it is necessary to fill up the via holes with a conducting resin past for forming the projection electrodes thereon, posing the problem of an increase in steps.
An object of the present invention is to provide a printed-circuit resin board having packaging projection electrodes which is highly flexible and resistant to an external stress to act on the projections so as to exhibit a highly adhesive strength to via holes formed in the printed-circuit board.
A further object of the present invention is to provide a method for producing a circuit board having such projection electrodes which are easily formed.
Accordingly, the above object can be attained by the provision of a printed-circuit board wherein projection electrodes composed of conductive paste projecting from the surface are connected integrally with the via holes conductors filled with the conductive paste. The via hole are formed in insulating resin substrates to make up the board.
Particularly, the present invention resides in a printed-circuit board in which via hole conductors composed of a conducting paste is formed in the insulating resin substrate. The printed-circuit board is characterized in that the projection electrodes are produced by curing a conducting paste and integrated with, and secured to, the via hole conductors, projecting from the surface of the substrate.
The present invention includes the printed-circuit board on which such projection electrodes connected integrally with the via holes and projected from the front surface and the back face of the substrate.
In the invention, because the projection electrode and the via hole conductor disposed on the printed-circuit board are integrally formed of the same material, the adhesive strength between the both is large and variations in the strength of the projection electrodes can be reduced.
In the invention, the use of the same conducting paste as the via hole conductor enables it possible to form the via hole conductor and the projection electrode simultaneously, which is effective in simplifying the process and reducing the product cost.
According to a further aspect of the present invention, there is provided a method for producing a printed-circuit board comprising filling, with an conducting paste, the via holes in a prepreg provided with a parting film, pressing and heating the prepreg to cure it into a board, and in succession peeling the parting film form the board to expose the projections. In the invention, the projection electrodes consist of the cured conducting paste and have an appropriate height from the board corresponding to the thickness of the parting film.
Preferably the projection electrodes are formed on the position corresponding to the via holes by interposing and pressing the substrate using molds with concave portions for forming the projections. The projection electrodes can be shaped into a desired and accurate finished form by utilizing the molds.
Preferably porous prepregs is used for the insulating resin substrate of the printed circuit board. This has an advantage in that the conductive paste with which the via holes are filled is allowed to project from the surface of the substrate with ease and at the same time the projection electrodes can be formed in one step of heating the prepreg under pressure.